Technical Field
The present disclosure relates to an optical lens module and an imaging apparatus. More particularly, the present disclosure relates to an optical lens module and an imaging apparatus which is applicable to portable electronic devices.
Description of Related Art
Due to the popularity of personal electronic products and mobile communication products having camera functionalities, such as smart phones and tablet personal computers, the demand for compact imaging apparatuses has been increasing, and the requirements for high resolution and image quality of present compact imaging apparatuses increase significantly.
With the trends of the high-pixels camera functionalities of personal electronic products and mobile communication products, the dimension of the imaging apparatus has been increasing such as the diameter of the lens element closest to the image surface of the imaging apparatus reaching 6 mm. On the other hand, the back focal length of the imaging apparatus is still kept short. Hence, more and more non-imaging light is reflected from the surface of the IR-cut filter (infrared-cut filter) to the surfaces of the lens elements, as well as the incident angles on the surfaces of the lens elements are usually greater than the critical angle of the total reflection thereof, so that the reflection from the surfaces of the lens elements to the image surface of the imaging apparatus has been increased, and it results in the ghost image on the image surface of the imaging apparatus.
FIG. 1 is a schematic view of a conventional imaging apparatus 9000. According to the conventional imaging apparatus 9000, the non-imaging light Lia would be reflected from the object-side surface 9601 of the IR-cut filter 9600 to the inner wall 9521 of the plastic barrel 9520, and would be reflected from the inner wall 9521 of the plastic barrel 9520 to the object-side surface 9511 of the lens element 9510 as the non-imaging light Loa, wherein the non-imaging light Loa is attenuated little from the non-imaging light Lia, so that the strength of the non-imaging light Loa is still approach to the strength of the non-imaging light Lia.
As well as the incident angle θ on the object-side surface 9511 of the lens element 9510 is greater than the critical angle θc of the total reflection thereof, so that the reflection from the object-side surface 9511 of the lens element 9510 to the image surface 9700 of the conventional imaging apparatus 9000 results in the ghost image on the image surface 9700 of the conventional imaging apparatus 9000.
If the index of refraction of the lens element 9510 in d-line reference wavelength (587.6 nm) is 1.535, then the critical angle θ, of the total reflection of the lens element 9510 is 40.65 degrees as the following:
      θ    c    =                              sin          1                ⁡                  (                                    N              air                                      N              lens                                )                    ⁢                        sin          1                ⁡                  (                      1            1.535                    )                      =          40.65      ⁢                          ⁢              degrees        .            
Herein, Nair is the index of refraction of air, which is 1 by assumption, Nlens is the index of refraction of the lens element 9510. Furthermore, with the greater diameter of the lens element 9510, more non-imaging light with incident angle above 40.65 degrees has been reflected.
Another instance, if the index of refraction of the lens element in d-line reference wavelength (587.6 nm) is 1.544, then the critical angle θ, of the total reflection of the lens element is 40.37 degrees as the following:
      θ    c    =                              sin                      -            1                          ⁡                  (                                    N              air                                      N              lens                                )                    ⁢                        sin                      -            1                          ⁡                  (                      1            1.544                    )                      =          40.37      ⁢                          ⁢              degrees        .            
However, the aforementioned problems are difficult to solve by the conventional coating of the lens elements. Therefore, the conventional coating cannot satisfy the requirements of reducing the reflection resulting in the ghost image on the image surface of the imaging apparatus.
In view of the above, how to reduce the total reflection of non-imaging light resulting in the ghost image on the image surface for enhancing the image quality of compact imaging apparatuses has become one of the important subjects.